Gravitational wave echos from physical black holes

Gravitational wave echos from the coalescence of black hole binaries are often viewed as signals beyond general relativity or standard model. In this work, we show that these echos are inevitable in the black holes coalescence described by standard general relativity. This is because it is the physical black holes formed through gravitational collapse serve as the true description of astronomical black holes. For physical black holes, only their asymptotic structure before the horizon forms are detectible to the outside probes. Here, by investigating the scattering of a gravitational wave burst on a physical black hole and pay special attention to the echos in the waveform, we uncover distinct features of the echos both in the time and frequency domains.